1. Field of the Invention
The present invention relates to a dynamic quantity detection device that detects a dynamic quantity.
2. Description of the Related Art
Conventionally, as a dynamic quantity detection device that detects a dynamic quantity such as acceleration and pressure, various types of dynamic quantity detection device including a capacitance type and a piezoelectric type are disclosed, for example, in US 2003/0164043 A1 (corresponding to JP-A-2003-248016) and JP-A-2007-171057. A capacitance-type dynamic quantity detection device includes a sensor element whose capacitance changes with the dynamic quantity, converts the capacitance of the sensor element into a voltage signal with a C-V converter circuit, and outputs the converted signal as a sensor signal. A piezoelectric-type dynamic quantity detection device includes a piezoelectric element as a sensor element.
These kind of dynamic quantity detection devices are used, for example, in an airbag system of a vehicle, as an acceleration sensor or a pressure sensor for detecting a collision of the vehicle. In an air bag system disclosed in JP-A-2004-284422, when a sensor signal from an acceleration sensor is greater than a threshold value, it is determined that the vehicle is in danger of collision, and a control device including a microcomputer is activated. In a normal state where the sensor signal is not greater than the threshold value, an operation of the control device is stopped (i.e., sleep) so as to save a power consumption of the control device and achieve a power saving of the airbag system.
In a case where a sensor signal from a dynamic quantity detection device is input to a control device like the above-described airbag system, there is an issue that when a signal level of the sensor signal is greater than a dynamic range of an input system of the control device, the sensor signal is clamped to the maximum value of the dynamic range, a dynamic quantity is difficult to be detected at the control device when the sensor signal is large, and a collision of the vehicle cannot be predicted or determined with accuracy.
If the dynamic range of the sensor signal is adjusted in such a manner that the maximum value of the sensor signal output from the dynamic quantity detection device corresponds to the dynamic range of the input system of the control device in order to solve the above-described issue, the sensor signal may be too small at a range where the dynamic quantity is required to be detected at the control device with accuracy, and a detection accuracy of the dynamic quantity is reduced.
Thus, the dynamic quantity detection device normally includes a signal processing circuit, such as an amplifier circuit and a low pass filter, that processes the sensor signal obtained through the sensor element, adjusts the dynamic range of the sensor signal to the dynamic range of the input system, and largely changes the sensor signal at a region where a detection accuracy of the dynamic quantity is required.
In a control system including this kind of dynamic quantity detection device, in particular, in a control system including a plurality of dynamic quantity detection devices, the amount of power consumption of the whole system is increased due to the amount of power consumed at the dynamic quantity detection device. In order to achieve a power saving of the control system, it is required to reduce the power consumption of the dynamic quantity detection device as well as the power consumption of the control device.
The dynamic quantity detection device detects the dynamic quantity, such as acceleration and pressure, applied to a detection object, such as a vehicle, and outputs the sensor signal. In addition, the dynamic quantity detection device needs to output the sensor signal without response delay when the dynamic quantity changes. Thus, conventionally, an internal circuit such as the signal processing circuit is constantly activated.
As a result, in a conventional dynamic quantity detection device, a power consumption of the internal circuit such as the signal processing circuit is difficult to be reduced sufficiently, and thereby the power saving of the control system including the dynamic quantity control device is restricted.